This invention relates to a low noise telephone line interface for data access arrangements (DAA). Specifically, it relates to a line powered DAA having significantly improved linearity and accuracy over the prior art.
The telephone lines to a residence in the United States and elsewhere can have common mode voltages of over 100V, and the FCC requires the telephone lines to be isolated from any electric main powered device (such as a PC) connected to the telephone lines (through a modem for example) to prevent damage to the telephone network. 47 CFR 68.302,4 (10-1-97 Edition). A data access arrangement (DAA) is specified by the FCC to isolate the telephone lines from electric main powered devices, such as illustrated in FIG. 3. Id. Since the voice band modem signal is limited to the 100 to 3600 Hz band, a DAA can be constructed using a transformer which operates as a bandpass filter to isolate the electric main powered device from the telephone lines.
A smaller size and potentially lower cost solution uses active circuits to communicate with the central telephone office and various modulation techniques to couple the DAA through small capacitors to the PC.
FIG. 4 shows a known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/028,061 filed on Feb. 26, 1998, entitled Low Noise Line Powered DAA With Feedback, assigned to the same assignee as the present application, and is incorporated herein by reference. The circuit is designed in low voltage CMOS technology and can handle only a small amount of voltage. The main function of the circuit is to take the incoming current, ILINE, supplied by the telephone company and modulate it with a data signal developed by processing a differential data signal source, VD, with a line modulator so as to place the data signal on the telephone line. The prior art circuit uses transistor Q1 as a line modulator, and contains a shunt regulator in series with the line modulator Q1. A sense resistor RS1 is placed in series between the line modulator Q1 and the shunt regulator to monitor the current through the shunt regulator.
The prior art circuit depicted in FIG. 4 works by monitoring the current through sense resistor RS1 with a feedback loop around the amplifier A. Resistors RTI and RB1 sense the differential voltage across RS1. By setting RT1=RB1, the current through RT1 and RB1 will accurately model the current through RS1. The desired signal to be modulated is introduced by a differential signal source VD. The differential signal is created by adding signal VD/2 to VCM to create VP and subtracting VD/2 from VCM to create VN. This differential signal then drives the input resistors RIP and RIN to provide a differential signal input current. The generation of the differential signal current is well known in prior art and will not be further discussed herein. The control amplifier operates to force the current through resistor RS1 to equal the desired signal current by regulating transistor Q2 to control the base of transistor Q1, which in turn regulates the current through the source-emitter path of transistor Q1 and thereby through resistor RS1. In this circuit, the collector current of transistor Q1 is well controlled by the control amplifier A. However, this arrangement incurs a degree of error which is problematic for new communication devices such as high speed data modems.
Ideally, the current through RS1 would equal the current, ILINE, introduced to the system by the telephone company. An error exists in the prior art line modulation device of FIG. 4 due to the inclusion of only part of the total current ILINE in RS1. The current from the telephone company is introduced to the system through the emitter of transistor Q1 (hereinafter xe2x80x9cIE1xe2x80x9d). In the prior art circuit depicted in FIG. 4, IE1 is equal to ILINE, the resistances of RT1 and RB1 are a couple hundred thousand ohms, and the resistance of RS1 is 10-20 ohms. Because of the relatively high level of resistance of RT1 and RB1, the current that flows through RT1 and RB1 can be neglected in the circuit analysis. As current flows through the circuit, IE1 is divided into the transistor Q1 base current (hereinafter xe2x80x9cIB1xe2x80x9d) and the transistor Q1 collector current (hereinafter xe2x80x9cIC1xe2x80x9d). The collector current IC1 through the resistor RS1 is used by amplifier A in a feedback loop to modulate the desired signal onto ILINE. Since the current IB1 is outside the feedback loop, an error term in the amount of IB1 is introduced to the circuit, that is, IC1 through resistor RS1 is not equal to ILINE, but is equal to IE1xe2x88x92IB1 or ILINExe2x88x92IB1.
An additional problem arises from IB1 being outside the amplifier feedback path. Since IC1 and IB1 are related by the Beta of Q1, and the Beta of a transistor is a function of the actual signal level, the error term introduced by not accounting for current IB1 in the feedback loop is signal dependent. Signal dependent error terms are a source of harmonic distortion which is problematic for communication devices. In order for current 56 k modems (V.90 standard) to function, a signal to distortion ratio greater than 80 dB is needed. Unfortunately, due to the error term introduced by neglecting IB1, the circuit of FIG. 4 can provide a signal to distortion ratio of only about 75 dB even when high quality components are utilized.
The present invention proposes a novel method and apparatus for increasing the signal to distortion ratio in a line powered telephone line interface or data access arrangement (DAA). The present invention accomplishes this task by reducing the amount of error which is inherent to the prior art circuit design by adapting the circuit of FIG. 4 to incorporate more of the total line current supplied by the telephone company into the feedback circuit. The invention modifies the prior art circuit by inserting an additional sense resistor to sense the current that is flowing through the line modulator but not through the shunt regulator.
According to one aspect of the present invention, the prior art is improved upon by inserting an additional sense resistor in series with the emitter output of transistor Q2, found within the line modulator. In the prior art circuit, the telephone line current ILINE enters the DAA through the emitter of transistor Q1. The current that is allowed to flow through Q1 results in a base current of Q1, IB1, and a collector current of Q1, IC1. The prior art only incorporates IC1 into the feedback path of the amplifier, leaving IB1 outside of the feedback path. Since transistor Q2 is electrically connected to the base of Q1, the current IB1 flows through transistor Q2. Therefore, the emitter of transistor Q2 is the path for most of the total circuit current ILINE which is not flowing through the prior art sense resistor or equivalently the shunt regulator.
In accordance with this aspect of the present invention, by serially connecting an additional sense resistor to the emitter output of transistor Q2 and including the current across the additional sense resistor, in addition to the current across the original sense resistor, in the feedback path, the amplifier can sense substantially all of ILINE within the circuit and process accordingly to remove noise and distortion. This arrangement results in an increase of the overall signal to distortion ratio in the circuit.